Nothing like revitalizing a dead thread...
Got a couple comments.
"A young jedi named Darth Vader, who was a pupil of mine until he turned to evil, helped the empire hunt down and destroy the Jedi Knights"
and the
"When I left you I was but the learner, now I am the master." (or some such)
These can easily be explained using the rules of the English language and exploring the situations the characters were in when they spoke.
The first quote, as I remember, is of OWK who has sat down Luke and his telling him a sort of BRIEF history. For those that are slow or blind, the keyword is BRIEF. Being concise is a top priority in good writing doctrines.
Now, Obi-Wan could have said, "A young jedi named Darth Vader, who was a pupil of mine, who then went on to complete the Jedi trials and became a Jedi Knight and turned to evil, helped the empire hunt down and destroy the Jedi Knights."
However, obviously this statement is awkward, especially from a "storyteller" point of view. While it's nice to add details, this one is rather mundane, and doesn't make for a good story nor does it help in the department of being brief and concise(something this sentence was not). So, in the spirit of good literature and story telling, this was left out.
The second quote can be seen in the same light. Darth Vader is about to engage Obi-Wan IN A SWORD FIGHT. I will further add that IT WAS TO THE DEATH.
Darth Vader obviously wanted to impress the fact that at some point Obi-Wan was his teacher. As far as the script goes, I can see that point being made very clear. It's short and easy to understand. Darth Vader could have engaged Obi-Wan in a full-length dissertation of the every step that it took from his Padawanship to his desertion of the Light, but under the circumstances(again, swordfight) it just wasn't necessary.
All these characters, in both instances, were trying to get across was that they were student/teacher. While if we take the text literally we come up with a contradiction, this contradiction can be overlooked by understanding what these characters were trying to get across. All these characters were trying to do was enlighten the audience as to a student/teacher bond they shared. They did so in short, concise sentences. People do this sort of paraphrasing all the time. As an example I will explain the start of my day... I woke up this morning, hopped into the shower, and then went to work. If you were to critically, and literally, analyze this statement, you will notice that I woke up, hopped into the shower(apparantly still wearing my PJ's as I did not mention taking those off), and then went to work(either in wet pajamas or naked, depending on whether or not you caught the fact that I didn't mention my undressing). However, you probably INFERRED that I had undressed before showering, and that I put work clothes on after that shower. People paraphrase all the time, it's part of daily life. Neither Obi-Wan or Darth Vader had to go to elaborate lengths to completely factualize their past. They simply spoke naturally, trying to convey a complicated message as concisely as possible. One of the beautiful things about the human mind is that it can interpret abstract thoughts and come to conclusions based on inferences. One should not need every single step, instance, or fact in any given pattern in order to come into understanding of that pattern. Leave the cold, hard logic to calculators and binary automatons. Learn to love the English language and the art of conciseness.